The present invention relates to an applied technique of an MR (Magnetic Resonance) phenomenon and, more particularly, to an MRI (Magnetic Resonance Imaging) system for imaging MR data of a specific nucleus spin in a specific slice of an object to be examined.
In an MRI system, a uniform static field HO is applied to an object to be examined (i.e., a patient), and a high-frequency (radiofrequency (RF)) rotating field (electromagnetic wave) is then applied thereto to excite an MR phenomenon. An electromagnetic wave generated by the MR phenomenon is detected, to acquire MR signals. Upon excitation of the MR phenomenon and/or acquisition of the MR signals, a strength gradient (magnetic gradient) according to deviation in a specific direction is added to the static field HO, thereby causing the acquired MR signals to include positional data. Various methods for exciting the MR phenomenon and/or acquiring the MR signals are used or have been proposed. The acquired MR signals are subjected to predetermined processing to obtain an image representing a distribution of the MR data in a specific slice of a patient. In order to obtain the magnetic gradient, inclined fields Gx, Gy, and Gz along orthogonal axes X, Y, and Z of a coordinate system having the direction of static field HO given by the Z axis are selectively used. The rotating field is normally applied in the form of excitation pulses (RF pulses), the envelope of which is represented by a pulse shape. As the RF pulses, 90.degree. pulses for changing the direction of magnetization of a nucleus spin through 90.degree., and/or 180.degree. pulses for changing it through 180.degree., are often used.
In most methods used for exciting the MR phenomenon and/or acquiring the MR signals, the imaging sequence for obtaining an MR image can be divided into an excitation sequence for exciting the MR phenomenon and a signal acquisition sequence following the excitation sequence. A detailed imaging sequence will now be described with reference to a spin echo imaging method.
90.degree. pulses as selective excitation pulses together with inclined field Gz along the Z axis are applied to a patient. Thereafter, inclined field Gy for encoding a phase is applied to the patient for a first predetermined period of time. After the first predetermined period of time has passed and time .tau. has elapsed from application of the 90.degree. pulses, 180.degree. pulses are applied to the patient. Thereafter, the MR signal is detected while applying inclined field Gx for a second predetermined period of time. After time .tau. has passed from the application of 180.degree. pulses, a spin echo is detected as the MR signal. In this case, the period from application of the 90.degree. pulses to that of the 180.degree. pulses corresponds to the period of the excitation sequence. The echo detection period after application of the 180.degree. pulses corresponds to the acquisition sequence period. After period TE, from application of the 90.degree. pulses, has passed the spin echo is detected. The imaging sequence consisting of the excitation and acquisition sequences is repeated for cycle TR. The signals acquired during the imaging sequence are two-dimensionally Fourier-transformed for the X-Y plane to obtain image data of a given slice. During a single imaging sequence, data corresponding to one slice can be aquiered. However, in general, the data obtained after a large number of imaging sequences are integrally mixed to obtain an image for one frame, for representing the slice more clearly. Different values of inclined field Gy for phase encoding are used for every repetitive imaging sequence, and the acquired data corresponding to the respective imaging sequences are discriminated by these values of field Gy.
In the MRI system, when an MR image of a thorax or an abdomen of a patient is to be obtained, if there is any movement of a patient, such as respiratory movement or heartbeats during the MR signal acquisition sequence, the resultant MR image is blurred. In order to prevent such blurring, the imaging operation can be performed in synchronism with respiratory movement or heartbeats of a patient. In a conventional system, the imaging operation is performed simply in synchronism with respiratory movement or heartbeats. For this reason, when the imaging operation is performed in synchronism with respiratory movement having a relatively long interval, the imaging period is prolonged, and it takes a great deal of time to obtain the necessary MR image data. When the imaging operation is performed in synchronism with the heartbeats, the imaging operation cannot cope with abnormal movement such as extrasystole or arrhythmia.